1. Field of the Invention
The present invention relates to an apparatus for measuring the time constant of the direct-axis damper of a synchronous machine among machine parameters needed for the computation in its control operation based on vector control, the measurement being implemented in the stage of the stand-alone machine test during assembly of the machine in the factory.
2. Description of the Prior Art
There is no test method available for directly measuring the time constant of the direct-axis damper of a synchronous machine. For example, a publication "Procedure of Testing a Synchronous Machine, Part 1 through Part 4", published by The Institute of Electrical Engineers of Japan, does not describe any technique for directly measuring the time constant of the direct-axis damper of a synchronous machine. A method of indirect measurement is conceivable based upon treating the damper as a secondary conductor of an induction motor, measuring the input power in a lock test, the equivalent resistance which matches the input power is calculated and the direct-axis damper time constant is finally obtained from the ratio of the equivalent resistance to the armature reaction inductance which is evaluated separately. However, a model of the synchronous machine used in the control computation, which is the inherent purpose, does not match model of the synchronous machine in the indirect measuring method based on the conversion of input power, and therefore the direct-axis damper time constant cannot be determined.
Moreover, the damper time constant needed in the control computation is the time constant in a microscopic sense resulting from the treatment of unbalanced multi-phase windings formed of multiple conductor bars embedded in the core of magnetic poles of the synchronous machine. Accordingly, evaluating the inductance and resistance sequentially and calculating their ratio for obtaining the time constant, which is the case relevant to the usual concentrated circuit parameter system, cannot be applied to the present situation.
Conventionaly, synchronous machines have been used as generators and motors operated at constant revolving speeds. These machines are provided with dampers for a purpose of suppressing speed fluctuation and for the additional purpose of producing a starting torque by utilizing it as a cage rotor of induction motor. In such circumstances of dampers, it has not been necessary to known the value of the damper time constant, and accordingly there has been no need of measuring the damper time constant directly.
Due to the conventional affair of measuring method intended for the evaluation of the direct-axis damper time constant of a synchronous machine as described above, it has not been possible for the implementation of high-accuracy control called "vector control" to assess the damper time constant as one of machine parameters used by the controller. On this account, it has been necessary to have iterative operation tests for the machine and controller in combination thereby to improve control performance by narrowing the assessment of the time constant, resulting unfavorably in an excessive expenditure of time and cost for this test step.
Another problem is that the accurate evaluation of the direct-axis damper time constant is required at the stage of testing a machine alone immediately after it has been assembled in the factory, so that the optimal value of the machine time constant, which has a great effect on the accuracy of control computation, is set to the controller in order to accomplish high-accuracy control.